High-quality (Q)-factor filters are widely implemented in communications and radar systems to reject out-of-band noise and interference while minimizing the attenuation of in-band signals. In order to achieve high Q factors, 3-D structures such as waveguide cavities, evanescent-mode cavities, dielectric resonators (DRs), nonplanar combline structures, quasi-planar electromagnetic bandgap (EBG) structures, and substrate integrated waveguides (SIW) can be used rather than low-Q planar transmission line structures. In addition, highly efficient antennas can improve signal-to-noise ratio (SNR) for receivers and reduce power consumption for transmitters. Traditionally, filters and antennas are separate from one another and are connected via standard 50-ohm ports such as coaxial connectors, which usually results in bulky structures, particularly for 3-D filters and antennas.
To avoid the coaxial connections, a vertical three-pole cavity filter can be integrated with a patch antenna inside a low-temperature cofired ceramic (LTCC) substrate. Such a filter and antenna can be designed separately using 50-ohm ports and then connected together through a slot-to-microstrip transition. The total loss of such system includes the addition of the individual losses from the filter, antenna, and the transition between the filter and the antenna. This transition causes significant connection losses and strongly detunes the filter response due to the antenna loading effect.
The integration of a coplanar waveguide (CPW) filter and a patch antenna without using 50-ohm transitions is also known. The filter and antenna can be co-designed as a single unit whereas the antenna works as both a resonator and a port of the filter. As a result, there are no transition losses and detuning effects in this filter/antenna system. Since this approach was based on a planar transmission line structure, equivalent circuit models were fairly easy to derive and use for optimizing the filter/antenna system. However, the achievable Q factors using this approach are very limited (e.g., generally <200).